Process for improved coupling of rubbery polymers

ABSTRACT

Rubbery polymers made by anionic polymerization can be coupled with tin halides or silicon halides to improve the characteristics of the rubber for use in some applications, such as tire treads. In cases where the rubbery polymer was synthesized utilizing a polar modifier it is difficult to attain a high level of coupling. This invention is based upon the unexpected finding that coupling efficiency can be significantly improved by conducting the coupling reaction in the presence of a lithium salt of a saturated aliphatic alcohol, such as lithium t-amylate. This invention discloses a process for coupling a living rubbery polymer that comprises reacting the living rubbery polymer with coupling agent selected from the group consisting of tin halides and silicon halides in the presence of a lithium salt of a saturated aliphatic alcohol. The lithium salt of the saturated aliphatic alcohol can be added immediately prior to the coupling reaction or it can be present throughout the polymerization and coupling process. Lithium t-amylate reacts with water to form t-amyl alcohol during steam stripping. Since t-amyl alcohol forms an azeotrope with hexane, it co-distills with hexane and can contaminate recycle feed streams. This problem of recycle stream contamination can be solved by using metal salts of cyclic alcohols that do not co-distill with hexane or form compounds during steam stripping which co-distill with hexane. Thus, the use of metal salts of cyclic alcohols is preferred for this reason and because they are considered to be environmentally safe.

This is divisional of U.S. patent application Ser. No. 09/461,653, filedon Dec. 14, 1999 now U.S. Pat. No. 6,489,403.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to exhibit good tractioncharacteristics on both dry and wet surfaces. However, it hastraditionally been very difficult to improve the tractioncharacteristics of a tire without compromising its rolling resistanceand tread wear. Low rolling resistance is important because good fueleconomy is virtually always an important consideration. Good tread wearis also an important consideration because it is generally the mostimportant factor that determines the life of the tire.

The traction, tread wear, and rolling resistance of a tire is dependentto a large extent on the dynamic viscoelastic properties of theelastomers utilized in making the tire tread. In order to reduce therolling resistance of a tire, rubbers having a high rebound havetraditionally been utilized in making the tire's tread. On the otherhand, in order to increase the wet skid resistance of a tire, rubberswhich undergo a large energy loss have generally been utilized in thetire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads. For instancevarious mixtures of styrene-butadiene rubber and polybutadiene rubberare commonly used as a rubber material for automobile tire treads.However, such blends are not totally satisfactory for all purposes.

The inclusion of styrene-butadiene rubber (SBR) in tire treadformulations can significantly improve the traction characteristics oftires made therewith. However, styrene is a relatively expensive monomerand the inclusion of SBR is tire tread formulations leads to increasedcosts.

Carbon black is generally included in rubber compositions which areemployed in making tires and most other rubber articles. It is desirableto attain the best possible dispersion of the carbon black throughoutthe rubber to attain optimized properties. It is also highly desirableto improve the interaction between the carbon black and the rubber. Byimproving the affinity of the rubber compound to the carbon black,physical properties can be improved. Silica can also be included in tiretread formulations to improve rolling resistance.

U.S. Pat. No. 4,843,120 discloses that tires having improved performancecharacteristics can be prepared by utilizing rubbery polymers havingmultiple glass transition temperatures as the tread rubber. Theserubbery polymers having multiple glass transition temperatures exhibit afirst glass transition temperature which is within the range of about−110° C. to −20° C. and exhibit a second glass transition temperaturewhich is within the range of about −50° C. to 0° C. According to U.S.Pat. No. 4,843,120, these polymers are made by polymerizing at least oneconjugated diolefin monomer in a first reaction zone at a temperatureand under conditions sufficient to produce a first polymeric segmenthaving a glass transition temperature which is between −110° C. and −20°C. and subsequently continuing said polymerization in a second reactionzone at a temperature and under conditions sufficient to produce asecond polymeric segment having a glass transition temperature which isbetween −20° C. and 20° C. Such polymerizations are normally catalyzedwith an organolithium catalyst and are normally carried out in an inertorganic solvent.

U.S. Pat. No. 5,137,998 discloses a process for preparing a rubberyterpolymer of styrene, isoprene, and butadiene having multiple glasstransition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises:terpolymerizing styrene, isoprene and 1,3-butadiene in an organicsolvent at a temperature of no more than about 40° C. in the presence of(a) at least one member selected from the group consisting oftripiperidino phosphine oxide and alkali metal alkoxides and (b) anorganolithium compound.

U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outercircumferential tread where said tread is a sulfur cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about −10° C. to about −40° C.and, further the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubberwhich is particularly valuable for use in making truck tire treads whichexhibit improved rolling resistance and tread wear characteristics, saidrubber being comprised of repeat units which are derived from about 5weight percent to about 20 weight percent styrene, from about 7 weightpercent to about 35 weight percent isoprene, and from about 55 weightpercent to about 88 weight percent 1,3-butadiene, wherein the repeatunits derived from styrene, isoprene and 1,3-butadiene are inessentially random order, wherein from about 25% to about 40% of therepeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40% to about 60% of the repeatunits derived from the 1,3-butadiene are of the trans-microstructure,wherein from about 5% to about 25% of the repeat units derived from the1,3-butadiene are of the vinyl-microstructure, wherein from about 75% toabout 90% of the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10% to about 25% of the repeatunits derived from the isoprene are of the 3,4-microstructure, whereinthe rubber has a glass transition temperature which is within the rangeof about −90° C. to about −70° C., wherein the rubber has a numberaverage molecular weight which is within the range of about 150,000 toabout 400,000, wherein the rubber has a weight average molecular weightof about 300,000 to about 800,000, and wherein the rubber has aninhomogeneity which is within the range of about 0.5 to about 1.5.

U.S. Pat. No. 5,239,009 reveals a process for preparing a rubberypolymer which comprises: (a) polymerizing a conjugated diene monomerwith a lithium initiator in the substantial absence of polar modifiersat a temperature which is within the range of about 5° C. to about 100°C. to produce a living polydiene segment having a number averagemolecular weight which is within the range of about 25,000 to about350,000; and (b) utilizing the living polydiene segment to initiate theterpolymerization of 1,3-butadiene, isoprene, and styrene, wherein theterpolymerization is conducted in the presence of at least one polarmodifier at a temperature which is within the range of about 5° C. toabout 70° C. to produce a final segment which is comprised of repeatunits which are derived from 1,3-butadiene, isoprene, and styrene,wherein the final segment has a number average molecular weight which iswithin the range of about 25,000 to about 350,000 The rubbery polymermade by this process is reported to be useful for improving the wet skidresistance and traction characteristics of tires without sacrificingtread wear or rolling resistance.

U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymers havinghigh vinyl contents which can reportedly be employed in building tireswhich have improved traction, rolling resistance, and abrasionresistance. These high vinyl isoprene-butadiene rubbers are synthesizedby copolymerizing 1,3-butadiene monomer and isoprene monomer in anorganic solvent at a temperature which is within the range of about −10□C. to about 100□ C. in the presence of a catalyst system which iscomprised of (a) an organoiron compound, (b) an organoaluminum compound,(c) a chelating aromatic amine, and (d) a protonic compound; wherein themolar ratio of the chelating amine to the organoiron compound is withinthe range of about 0.1:1 to about 1:1, wherein the molar ratio of theorganoaluminum compound to the organoiron compound is within the rangeof about 5:1 to about 200:1, and herein the molar ratio of the protoniccompound to the organoaluminum compound is within the range of about0.001:1 to about 0.2:1.

U.S. Pat. No. 5,405,927 discloses an isoprene-butadiene rubber which isparticularly valuable for use in making truck tire treads, said rubberbeing comprised of repeat units which are derived from about 20 weightpercent to about 50 weight percent isoprene and from about 50 weightpercent to about 80 weight percent 1,3-butadiene, wherein the repeatunits derived from isoprene and 1,3-butadiene are in essentially randomorder, wherein from about 3% to about 10% of the repeat units in saidrubber are 1,2-polybutadiene units, wherein from about 50% to about 70%of the repeat units in said rubber are 1,4-polybutadiene units, whereinfrom about 1% to about 4% of the repeat units in said rubber are3,4-polyisoprene units, wherein from about 25% to about 40% of therepeat units in the polymer are 1,4-polyisoprene units, wherein therubber has a glass transition temperature which is within the range ofabout −90° C. to about −75° C., and wherein the rubber has a Mooneyviscosity which is within the range of about 55 to about 140.

U.S. Pat. No. 5,654,384 discloses a process for preparing high vinylpolybutadiene rubber which comprises polymerizing 1,3-butadiene monomerwith a lithium initiator at a temperature which is within the range ofabout 5° C. to about 100° C. in the presence of a sodium alkoxide and apolar modifier, wherein the molar ratio of the sodium alkoxide to thepolar modifier is within the range of about 0.1:1 to about 10:1; andwherein the molar ratio of the sodium alkoxide to the lithium initiatoris within the range of about 0.05:1 to about 10:1. By utilizing acombination of sodium alkoxide and a conventional polar modifier, suchas an amine or an ether, the rate of polymeriztion initiated withorganolithium compounds can be greatly increased with the glasstransition temperature of the polymer produced also being substantiallyincreased. The rubbers synthesized using such catalyst systems alsoexhibit excellent traction properties when compounded into tire treadformulations. This is attributable to the unique macrostructure (randombranching) of the rubbers made with such catalyst systems.

U.S. Pat. No. 5,620,939, U.S. Pat. No. 5,627,237, and U.S. Pat. No.5,677,402 also disclose the use of sodium salts of saturated aliphaticalcohols as modifiers for lithium initiated solution polymerizations.Sodium t-amylate is a preferred sodium alkoxide by virtue of itsexceptional solubility in non-polar aliphatic hydrocarbon solvents, suchas hexane, which are employed as the medium for such solutionpolymerizations. However, using sodium t-amylate as the polymerizationmodifier in commercial operations where recycle is required can lead tocertain problems. These problems arise due to the fact that sodiumt-amylate reacts with water to form t-amyl alcohol during steamstripping in the polymer finishing step. Since t-amyl alcohol forms anazeotrope with hexane, it co-distills with hexane and thus contaminatesthe feed stream.

Tire rubbers which are prepared by anionic polymerization are frequentlycoupled with a suitable coupling agent, such as a tin halide, to improvedesired properties. Tin-coupled polymers are known to improve treadwearand to reduce rolling resistance when used in tire tread rubbers. Suchtin-coupled rubbery polymers are typically made by coupling the rubberypolymer with a tin coupling agent at or near the end of thepolymerization used in synthesizing the rubbery polymer. In the couplingprocess, live polymer chain ends react with the tin coupling agentthereby coupling the polymer. For instance, up to four live chain endscan react with tin tetrahalides, such as tin tetrachloride, therebycoupling the polymer chains together.

The coupling efficiency of the tin coupling agent is dependant on manyfactors, such as the quantity of live chain ends available for couplingand the quantity and type of polar modifier, if any, employed in thepolymerization. For instance, tin coupling agents are generally not aseffective in the presence of polar modifiers. However, polar modifierssuch as tetramethylethylenediamine, are frequently used to increase theglass transition temperature of the rubber for improved properties, suchas improved traction characteristics in tire tread compounds. Couplingreactions that are carried out in the presence of polar modifierstypically have a coupling efficiency of about 50-60% in batch processes.Lower coupling efficiencies are typically attained in continuousprocesses.

Each tin tetrahalide molecule or silicon tetrahalide molecule is capableof reacting with up to four live polymer chain ends. However, sinceperfect stoichiometry is difficult to attain, some of the tin halidemolecules often react with less than four live polymer chain ends. Theclassical problem is that if more than a stoichiometric amount of thetin halide coupling agent is employed, then there will be aninsufficient quantity of live polymer chain ends to totally react withthe tin halide molecules on a four-to-one basis. On the other hand, ifless than a stoichiometric amount of the tin halide coupling agent isadded, then there will be an excess of live polymer chain ends and someof the live chain ends will not be coupled. It is accordingly importantfor the stoichiometry to be exact and for all to the living polymerchain-ends to react with the coupling agent.

Conventional tin coupling results in the formation of a coupled polymerthat is essentially symmetrical. In other words, all of the polymer armson the coupled polymer are of essentially the same chain length. All ofthe polymer arms in such conventional tin-coupled polymers areaccordingly of essentially the same molecular weight. This results insuch conventional tin-coupled polymers having a low polydispersity. Forinstance, conventional tin-coupled polymers normally having a ratio ofweight average molecular weight to number average molecular weight whichis within the range of about 1.01 to about 1.1

U.S. Pat. No. 5,486,574 discloses dissimilar arm asymmetric radical orstar block copolymers for adhesives and sealants. U.S. Pat. No.5,096,973 discloses ABC block copolymers based on butadiene, isopreneand styrene and further discloses the possibility of branching theseblock copolymers with tetrahalides of silicon, germanium, tin or lead.

SUMMARY OF THE INVENTION

It has been unexpectedly found that coupling efficiency can besignificantly improved by conducting the coupling reactions in thepresence of a lithium salt of a saturated aliphatic alcohol, such aslithium t-amylate. In the alternative coupling efficiency can also beimproved by conducting the coupling reaction in the presence of alithium halide, or a lithium phenoxide.

This invention discloses a process for coupling a living rubbery polymerthat comprises reacting the living rubbery polymer with coupling agentselected from the group consisting of tin halides and silicon halides inthe presence of a lithium salt of a saturated aliphatic alcohol. Thelithium salt of the saturated aliphatic alcohol can be added immediatelyprior to the coupling reaction or it can be present throughout thepolymerization and coupling process.

Many metal salts of saturated aliphatic alcohols, react with water toproduce alcohols during steam stripping. For instance, lithium t-amylatecan react with water to produce t-amyl alcohol during steam stripping.Since t-amyl alcohol forms an azeotrope with hexane, it co-distills withhexane and can contaminate recycle feed streams. This problem of recyclestream contamination can be solved by using metal salts of cyclicalcohols that do not co-distill with hexane or form compounds duringsteam stripping which co-distill with hexane. Thus, the use of metalsalts of cyclic alcohols is preferred because they solve the problem ofrecycle stream contamination and are considered to be environmentallysafe. Lithium mentholate is a highly preferred lithium salt of a cyclicalcohol that can be used in the practice of this invention.

The present invention further discloses a process for coupling a livingrubbery polymer that comprises reacting the living rubbery polymer witha coupling agent selected from the group consisting of tin halides andsilicon halides in the presence of a member selected from the groupconsisting of lithium halides and lithium phenoxides.

The subject invention also reveals a stabilized lithium initiator systemwhich is comprised of (1) an alkyl lithium compound selected from thegroup consisting of secondary alkyl lithium compounds and tertiary alkyllithium compounds, (2) a lithium salt of a saturated aliphatic alcohol,and (3) a hydrocarbon solvent.

DETAILED DESCRIPTION OF THE INVENTION

Virtually any type of rubbery polymer prepared by anionic polymerizationcan be coupled in accordance with this invention. In fact, thetechniques of this invention can be used to couple virtually any type ofrubbery polymer synthesized by anionic polymerization. The rubberypolymers that can be coupled will typically be synthesized by a solutionpolymerization technique utilizing an organolithium compound as theinitiator. These rubbery polymers will accordingly normally contain a“living” lithium chain end.

The polymerizations employed in synthesizing the living rubbery polymerswill normally be carried out in a hydrocarbon solvent. Such hydrocarbonsolvents are comprised of one or more aromatic, paraffinic orcycloparaffinic compounds. These solvents will normally contain fromabout 4 to about 10 carbon atoms per molecule and will be liquid underthe conditions of the polymerization. Some representative examples ofsuitable organic solvents include pentane, isooctane, cyclohexane,methylcyclohexane, isohexane, n-heptane, n-octane, n-hexane, benzene,toluene, xylene, ethylbenzene, diethylbenzene, isobutylbenzene,petroleum ether, kerosene, petroleum spirits, petroleum naphtha, and thelike, alone or in admixture.

In the solution polymerization, there will normally be from 5 to 30weight percent monomers in the polymerization medium. Suchpolymerization media are, of course, comprised of the organic solventand monomers. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 25 weight percent monomers. It is generallymore preferred for the polymerization medium to contain 15 to 20 weightpercent monomers.

The rubbery polymers that are coupled in accordance with this inventioncan be made by the homopolymerization of a conjugated diolefin monomeror by the random copolymerization of a conjugated diolefin monomer witha vinyl aromatic monomer. It is, of course, also possible to make livingrubbery polymers that can be coupled by polymerizing a mixture ofconjugated diolefin monomers with one or more ethylenically unsaturatedmonomers, such as vinyl aromatic monomers. The conjugated diolefinmonomers which can be utilized in the synthesis of rubbery polymerswhich can be coupled in accordance with this invention generally containfrom 4 to 12 carbon atoms. Those containing from 4 to 8 carbon atoms aregenerally preferred for commercial purposes. For similar reasons,1,3-butadiene and isoprene are the most commonly utilized conjugateddiolefin monomers. Some additional conjugated diolefin monomers that canbe utilized include 2,3-dimethyl-1,3-butadiene, piperylene,3-butyl-1,3-octadiene, 2-phenyl-1,3-butadiene, and the like, alone or inadmixture.

Some representative examples of ethylenically unsaturated monomers thatcan potentially be synthesized into rubbery polymers which can becoupled in accordance with this invention include alkyl acrylates, suchas methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylateand the like; vinylidene monomers having one or more terminal CH2=CH—groups; vinyl aromatics such as styrene, α-methylstyrene, bromostyrene,chlorostyrene, fluorostyrene and the like; α-olefins such as ethylene,propylene, 1-butene and the like; vinyl halides, such as vinylbromide,chloroethane (vinylchloride), vinylfluoride, vinyliodide,1,2-dibromoethene, 1,1-dichloroethene (vinylidene chloride),1,2-dichloroethene and the like; vinyl esters, such as vinyl acetate;α,β-olefinically unsaturated nitriles, such as acrylonitrile andmethacrylonitrile; α,β-olefinically unsaturated amides, such asacrylamide, N-methyl acrylamide, N,N-dimethylacrylamide, methacrylamideand the like.

Rubbery polymers which are copolymers of one or more diene monomers withone or more other ethylenically unsaturated monomers will normallycontain from about 50 weight percent to about 99 weight percentconjugated diolefin monomers and from about 1 weight percent to about 50weight percent of the other ethylenically unsaturated monomers inaddition to the conjugated diolefin monomers. For example, copolymers ofconjugated diolefin monomers with vinylaromatic monomers, such asstyrene-butadiene rubbers which contain from 50 to 95 weight percentconjugated diolefin monomers and from 5 to 50 weight percentvinylaromatic monomers, are useful in many applications.

Vinyl aromatic monomers are probably the most important group ofethylenically unsaturated monomers which are commonly incorporated intopolydienes. Such vinyl aromatic monomers are, of course, selected so asto be copolymerizable with the conjugated diolefin monomers beingutilized. Generally, any vinyl aromatic monomer which is known topolymerize with organolithium initiators can be used. Such vinylaromatic monomers typically contain from 8 to 20 carbon atoms. Usually,the vinyl aromatic monomer will contain from 8 to 14 carbon atoms. Themost widely used vinyl aromatic monomer is styrene. Some examples ofvinyl aromatic monomers that can be utilized include styrene,1-vinylnaphthalene, 2-vinylnaphthalene, α-methylstyrene,4-phenylstyrene, 3-methylstyrene and the like.

Some representative examples of rubbery polymers which can be coupled inaccordance with this invention include polybutadiene, polyisoprene,styrene-butadiene rubber (SBR), α-methylstyrene-butadiene rubber,α-methylstyrene-isoprene rubber, styrene-isoprene-butadiene rubber(SIBR), styrene-isoprene rubber (SIR), isoprene-butadiene rubber (IBR),α-methylstyrene-isoprene-butadiene rubber andα-methylstyrene-styrene-isoprene-butadiene rubber. In cases where therubbery polymer is comprised of repeat units that are derived from twoor more monomers, the repeat units which are derived from the differentmonomers will normally be distributed in an essentially random manner.In other words, the rubbery polymer will not be a block copolymer.

The polymerizations employed in making the rubbery polymer are typicallyinitiated by adding an organolithium initiator to an organicpolymerization medium that contains the monomers. Such polymerizationsare typically carried out utilizing continuous polymerizationtechniques. In such continuous polymerizations, monomers and initiatorare continuously added to the organic polymerization medium with therubbery polymer synthesized being continuously withdrawn. Suchcontinuous polymerizations are typically conducted in a multiple reactorsystem.

The organolithium initiators which can be employed in synthesizingrubbery polymers which can be coupled in accordance with this inventioninclude the monofunctional and multifunctional types known forpolymerizing the monomers described herein. The multifunctionalorganolithium initiators can be either specific organolithium compoundsor can be multifunctional types which are not necessarily specificcompounds but rather represent reproducible compositions of regulablefunctionality.

The amount of organolithium initiator utilized will vary with themonomers being polymerized and with the molecular weight that is desiredfor the polymer being synthesized. However, as a general rule, from 0.01to 1 phm (parts per 100 parts by weight of monomer) of an organolithiuminitiator will be utilized. In most cases, from 0.01 to 0.1 phm of anorganolithium initiator will be utilized with it being preferred toutilize 0.025 to 0.07 phm of the organolithium initiator.

The choice of initiator can be governed by the degree of branching andthe degree of elasticity desired for the polymer, the nature of thefeedstock and the like. With regard to the feedstock employed as thesource of conjugated diene, for example, the multifunctional initiatortypes generally are preferred when a low concentration diene stream isat least a portion of the feedstock, since some components present inthe unpurified low concentration diene stream may tend to react withcarbon lithium bonds to deactivate initiator activity, thusnecessitating the presence of sufficient lithium functionality in theinitiator so as to override such effects.

The multifunctional initiators which can be used include those preparedby reacting an organomonolithium compounded with a multivinylphosphineor with a multivinylsilane, such a reaction preferably being conductedin an inert diluent such as a hydrocarbon or a mixture of a hydrocarbonand a polar organic compound. The reaction between the multivinylsilaneor multivinylphosphine and the organomonolithium compound can result ina precipitate which can be solubilized, if desired, by adding asolubilizing monomer such as a conjugated diene or monovinyl aromaticcompound, after reaction of the primary components. Alternatively, thereaction can be conducted in the presence of a minor amount of thesolubilizing monomer. The relative amounts of the organomonolithiumcompound and the multivinylsilane or the multivinylphosphine preferablyshould be in the range of about 0.33 to 4 moles of organomonolithiumcompound per mole of vinyl groups present in the multivinylsilane ormultivinylphosphine employed. It should be noted that suchmultifunctional initiators are commonly used as mixtures of compoundsrather than as specific individual compounds.

Exemplary organomonolithium compounds include ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, tert-octyllithium,n-eicosyllithium, phenyllithium, 2-naphthyllithium,4-butylphenyllithium, 4-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium and the like.

Exemplary multivinylsilane compounds include tetravinylsilane,methyltrivinylsilane, diethyldivinylsilane, di-n-dodecyldivinylsilane,cyclohexyltrivinylsilane, phenyltrivinylsilane, benzyltrivinylsilane,(3-ethylcyclohexyl) (3-n-butylphenyl)divinylsilane and the like.

Exemplary multivinylphosphine compounds include trivinylphosphine,methyldivinylphosphine, dodecyldivinylphosphine, phenyldivinylphosphine,cyclooctyldivinylphosphine and the like.

Other multifunctional polymerization initiators can be prepared byutilizing an organomonolithium compound, further together with amultivinylaromatic compound and either a conjugated diene ormonovinylaromatic compound or both. These ingredients can be chargedinitially, usually in the presence of a hydrocarbon or a mixture of ahydrocarbon and a polar organic compound as a diluent. Alternatively, amultifunctional polymerization initiator can be prepared in a two-stepprocess by reacting the organomonolithium compound with a conjugateddiene or monovinyl aromatic compound additive and then adding themultivinyl aromatic compound. Any of the conjugated dienes or monovinylaromatic compounds described can be employed. The ratio of conjugateddiene or monovinyl aromatic compound additive employed preferably shouldbe in the range of about 2 to 15 moles of polymerizable compound permole of organolithium compound. The amount of multivinylaromaticcompound employed preferably should be in the range of about 0.05 to 2moles per mole of organomonolithium compound.

Exemplary multivinyl aromatic compounds include 1,2-divinylbenzene,1,3-divinylbenzene, 1,4-divinylbenzene, 1,2,4-trivinylbenzene,1,3-divinylnaphthalene, 1,8-divinylnaphthalene,1,3,5-trivinylnaphthalene, 2,4-divinylbiphenyl, 3,5,4′-trivinylbiphenyl,m-diisopropenyl benzene, p-diisopropenyl benzene,1,3-divinyl-4,5,8-tributylnaphthalene and the like. Divinyl aromatichydrocarbons containing up to 18 carbon atoms per molecule arepreferred, particularly divinylbenzene as either the ortho, meta or paraisomer and commercial divinylbenzene, which is a mixture of the threeisomers, and other compounds, such as the ethylstyrenes, also is quitesatisfactory.

Other types of multifunctional initiators can be employed such as thoseprepared by contacting a sec- or tert-organomonolithium compound with1,3-butadiene, at a ratio of about 2 to 4 moles of the organomonolithiumcompound per mole of the 1,3-butadiene, in the absence of added polarmaterial in this instance, with the contacting preferably beingconducted in an inert hydrocarbon diluent, though contacting without thediluent can be employed if desired.

Alternatively, specific organolithium compounds can be employed asinitiators, if desired, in the preparation of polymers in accordancewith the present invention. These can be represented by R(Li)x wherein Rrepresents a hydrocarbyl radical containing from 1 to 20 carbon atoms,and wherein x is an integer of 1 to 4. Exemplary organolithium compoundsare methyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, 4-cyclohexylbutyllithium,dilithiomethane, 1,4-dilithiobutane, 1,10-dilithiodecane,1,20-dilithioeicosane, 1,4-dilithiocyclohexane, 1,4-dilithio-2-butane,1,8-dilithio-3-decene, 1,2-dilithio-1,8-diphenyloctane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4′-dilithiobiphenyl and the like.

The polymerization temperature utilized can vary over a broad range offrom about −20° C. to about 180° C. In most cases, a polymerizationtemperature within the range of about 30° C. to about 125° C. will beutilized. It is typically preferred for the polymerization temperatureto be within the range of about 45° C. to about 100° C. It is typicallymost preferred for the polymerization temperature to be within the rangeof about 60° C. to about 85° C. The pressure used will normally besufficient to maintain a substantially liquid phase under the conditionsof the polymerization reaction.

The polymerization is conducted for a length of time sufficient topermit substantially complete polymerization of monomers. In otherwords, the polymerization is normally carried out until high conversionsare attained. The polymerization is then terminated by the addition of atin halide and/or silicon halide. The tin halide and/or the siliconhalide are continuous added in cases where asymmetrical coupling isdesired. This continuous addition of tin coupling agent and/or thesilicon coupling agent is normally done in a reaction zone separate fromthe zone where the bulk of the polymerization is occurring. In otherwords, the coupling will typically be added only after a high degree ofconversion has already been attained. For instance, the coupling agentwill normally be added only after a monomer conversion of greater thanabout 90 percent has been realized. It will typically be preferred forthe monomer conversion to reach at least about 95 percent before thecoupling agent is added. As a general rule, it is most preferred for themonomer conversion to exceed about 98 percent before the coupling agentis added. The coupling agents will normally be added in a separatereaction vessel after the desired degree of conversion has beenattained. The coupling agents can be added in a hydrocarbon solution,e.g., in cyclohexane, to the polymerization admixture with suitablemixing for distribution and reaction.

The coupling agent will typically be a tin halide. The tin halide willnormally be a tin tetrahalide, such as tin tetrachloride, tintetrabromide, tin tetrafluoride or tin tetraiodide. However, tintrihalides can also optionally be used. Polymers coupled with tintrihalides having a maximum of three arms. This is, of course, incontrast to polymers coupled with tin tetrahalides which have a maximumof four arms. To induce a higher level of branching, tin tetrahalidesare normally preferred. As a general rule, tin tetrachloride is mostpreferred.

The silicon coupling agents that can be used will normally be silicontetrahalides, such as silicon tetrachloride, silicon tetrabromide,silicon tetrafluoride or silicon tetraiodide. However, silicontrihalides can also optionally be used. Polymers coupled with silicontrihalides having a maximum of three arms. This is, of course, incontrast to polymers coupled with silicon tetrahalides which have amaximum of four arms. To induce a higher level of branching, silicontetrahalides are normally preferred. As a general rule, silicontetrachloride is most preferred of the silicon coupling agents.

A combination of a tin halide and a silicon halide can optionally beused to couple the rubbery polymer. By using such a combination of tinand silicon coupling agents improved properties for tire rubbers, suchas lower hysteresis, can be attained. In such cases, the molar ratio ofthe tin halide to the silicon halide employed in coupling the rubberypolymer will normally be within the range of 20:80 to 95:5. The molarratio of the tin halide to the silicon halide employed in coupling therubbery polymer will more typically be within the range of 40:60 to90:10. The molar ratio of the tin halide to the silicon halide employedin coupling the rubbery polymer will preferably be within the range of60:40 to 85:15. The molar ratio of the tin halide to the silicon halideemployed in coupling the rubbery polymer will most preferably be withinthe range of 65:35 to 80:20.

Broadly, and exemplary, a range of about 0.01 to 4.5 milliequivalents oftin coupling agent (tin halide and silicon halide) is employed per 100grams of the rubbery polymer. It is normally preferred to utilize about0.01 to about 1.5 milliequivalents of the coupling agent per 100 gramsof polymer to obtain the desired Mooney viscosity. The larger quantitiestend to result in production of polymers containing terminally reactivegroups or insufficient coupling. One equivalent of tin coupling agentper equivalent of lithium is considered an optimum amount for maximumbranching. For instance, if a mixture tin tetrahalide and silicontetrahalide is used as the coupling agent, one mole of the couplingagent would be utilized per four moles of live lithium ends. In caseswhere a mixture of tin trihalide and silicon trihalide is used as thecoupling agent, one mole of the coupling agent will optimally beutilized for every three moles of live lithium ends. The coupling agentcan be added in a hydrocarbon solution, e.g., in cyclohexane, to thepolymerization admixture in the reactor with suitable mixing fordistribution and reaction.

In the practice of this invention, the coupling reaction is carried outin the presence of a lithium compound selected from the group consistingof lithium salts of a saturated aliphatic alcohol, a lithium halides,and lithium phenoxides. The molar ratio of the lithium compound to thepolar modifier will typically be within the range of about 0.01:1 to100:1. The molar ratio of the lithium compound to the polar modifierwill more typically be within the range of about 0.1:1 to 10:1. Themolar ratio of the lithium compound to the polar modifier willpreferably be within the range of about 0.4:1 to 2:1. The molar ratio ofthe lithium compound to the polar modifier will most preferably bewithin the range of about 0.7:1 to 1.4:1.

The lithium salt of the saturated aliphatic alcohol can be addedimmediately prior to coupling or it can be present during thepolymerization and coupling steps. The lithium compound can be addeddirectly as a salt of an saturated aliphatic alcohol or the salt can bemade “in-situ” by the addition of an saturated aliphatic alcohol. Forinstance, menthol can be added as a part of a lithium initiator systemand will react with organolithium compounds therein to form a lithiummentholate. It is generally preferred for the lithium salt of thesaturated aliphatic alcohol to be made by such an “in-situ” technique incommercial applications.

The lithium salt of the aliphatic alcohol can also be blended with theorganolithium compound prior to using it as an initiator. This offers asignificant advantage because it stabilizes the organolithium compound.Additionally, it makes the lithium salt of the aliphatic alcohol muchmore soluble in hydrocarbon solvents. For instance, secondary alkyllithium compounds, such as secondary-butyl lithium, and tertiary alkyllithium compounds, such as tertiary-butyl lithium, are extremelyunstable and typically must be used within 48 hours. However, it hasbeen found that salts of saturated aliphatic alcohols can be used tostabilize such secondary alkyl lithium compounds and tertiary alkyllithium compounds. For instance, secondary alkyl lithium compounds andtertiary alkyl lithium compounds can be stabilized with about 1 part byweight to about 100 parts by weight of a lithium salt of a saturatedaliphatic alcohol per 100 parts by weight of the secondary alkyl lithiumcompound or the tertiary alkyl lithium compound. Such compositions willtypically contain from about 10 parts by weight to about 50 parts byweight of the lithium salt of a saturated aliphatic alcohol per 100parts by weight of the secondary alkyl lithium compound or the tertiaryalkyl lithium compound. Such stabilized lithium initiator systems willtypically be dispersed in a hydrocarbon solvent.

The lithium salt of the saturated aliphatic alcohol will preferably be alithium alkoxide. Such lithium alkoxides are of the formula LiOR,wherein R is an alkyl group containing from about 2 to about 12 carbonatoms. The lithium alkoxide will typically contain from about 2 to about12 carbon atoms. It is generally preferred for the lithium alkoxide tocontain from about 3 to about 8 carbon atoms. It is generally mostpreferred for the lithium alkoxide to contain from about 4 to about 6carbon atoms. Lithium t-amyloxide (lithium t-pentoxide) is arepresentative example of a preferred lithium alkoxide that can beutilized in the process of this invention.

It should be noted that even small amounts of sodium alkoxides,potassium alkoxides, cesium alkoxides, or rubidium alkoxides result inundesirable side reactions, such as chain transfer. Thus, the couplingreactions of this invention are carried out in the absence of sodiumalkoxides, cesium alkoxides, rubidium alkoxides, and potassiumalkoxides. For instance, the presence of sodium salts of saturatedaliphatic alcohols, such as sodium alkoxides, causes an undesirable jumpin Mooney viscosity and interferes with improved coupling efficiency.

As has been explained it is preferred to utilize lithium salts of cyclicalcohols. The lithium salts of the cyclic alcohols that can bemono-cyclic, bi-cyclic or tri-cyclic. They can be substituted with 1 to5 hydrocarbon moieties and can also optionally contain hetero-atoms. Forinstance, the metal salt of the cyclic alcohol can be a metal salt of adi-alkylated cyclohexanol, such as 2-isopropyl-5-methylcyclohexanol or2-t-butyl-5-methylcyclohexanol. These salts are preferred because theyare soluble in hexane. Metal salts of disubstituted cyclohexanol arehighly preferred because they are soluble in hexane. Lithium mentholateis the most highly preferred metal salt of a cyclic alcohol that can beemployed in the practice of this invention. The metal salt of the cyclicalcohol can be prepared by reacting the cyclic alcohol directly with themetal or another metal source, such as cesium hydride, in an aliphaticor aromatic solvent.

After the coupling has been completed, a tertiary chelating alkyl1,2-ethylene diamine or a metal salt of a cyclic alcohol can optionallybe added to the polymer cement to stabilize the coupled rubbery polymer.The tertiary chelating amines that can be used are normally chelatingalkyl diamines of the structural formula:

wherein n represents an integer from 1 to about 6, wherein A representsan alkylene group containing from 1 to about 6 carbon atoms and whereinR′, R″, R′″ and R″″ can be the same or different and represent alkylgroups containing from 1 to about 6 carbon atoms. The alkylene group Ais the formula —(—CH₂—)_(m) wherein m is an integer from 1 to about 6.The alkylene group will typically contain from 1 to 4 carbon atoms (mwill be 1 to 4) and will preferably contain 2 carbon atoms. In mostcases, n will be an integer from 1 to about 3 with it being preferredfor n to be 1. It is preferred for R′, R″, R′″ and R″″ to representalkyl groups which contain from 1 to 3 carbon atoms. In most cases, R′,R′″, R′″ and R″″ will represent methyl groups.

A sufficient amount of the chelating amine or metal salt of the cyclicalcohol should be added to complex with any residual tin coupling agentremaining after completion of the coupling reaction.

In most cases, from about 0.01 phr (parts by weight per 100 parts byweight of dry rubber) to about 2 phr of the chelating alkyl 1,2-ethylenediamine or metal salt of the cyclic alcohol will be added to the polymercement to stabilize the rubbery polymer. Typically, from about 0.05 phrto about 1 phr of the chelating alkyl 1,2-ethylene diamine or metal saltof the cyclic alcohol will be added. More typically, from about 0.1 phrto about 0.6 phr of the chelating alkyl 1,2-ethylene diamine or themetal salt of the cyclic alcohol will be added to the polymer cement tostabilize the rubbery polymer.

After the polymerization, coupling, and optionally the stabilizationstep, has been completed, the coupled rubbery polymer can be recoveredfrom the organic solvent. The coupled rubbery polymer can be recoveredfrom the organic solvent and residue by means such as decantation,filtration, centrification and the like. It is often desirable toprecipitate the coupled rubbery polymer from the organic solvent by theaddition of lower alcohols containing from about 1 to about 4 carbonatoms to the polymer solution. Suitable lower alcohols for precipitationof the rubber from the polymer cement include methanol, ethanol,isopropyl alcohol, normal-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the asymmetricallytin-coupled rubbery polymer from the polymer cement also “kills” anyremaining living polymer by inactivating lithium end groups. After thecoupled rubbery polymer is recovered from the solution, steam-strippingcan be employed to reduce the level of volatile organic compounds in thecoupled rubbery polymer.

The coupled rubbery polymers that can be made by using the technique ofthis invention are comprised of a tin and/or silicon atoms having atleast three polydiene arms covalently bonded. In the case ofasymmetrically coupled rubbery polymers made by the technique of thisinvention at least one of the polydiene arms bonded to the tin atomsand/or the silicon atoms has a number average molecular weight of lessthan about 40,000, at least one of the polydiene arms bonded to the tinatoms and/or the silicon atoms has a number average molecular weight ofat least about 80,000. The ratio of the weight average molecular weightto the number average molecular weight of the asymmetrically coupledrubbery polymer will also normally be within the range of about 2 toabout 2.5.

The asymmetrically coupled rubbery polymers that can be made by theprocess of this invention contain stars of the structural formula:

wherein M represents silicon or tin, wherein R₁, R₂, R₃ and R₄ can bethe same or different and are selected from the group consisting ofalkyl groups and polydiene arms (polydiene rubber chains), with theproviso that at least three members selected from the group consistingof R₁, R₂, R₃ and R₄ are polydiene arms, with the proviso that at leastone member selected from the group consisting of R₁, R₂, R₃ and R₄ is alow molecular weight polydiene arm having a number average molecularweight of less than about 40,000, with the proviso that at least onemember selected from the group consisting of R₁, R₂, R₃ and R₄ is a highmolecular weight polydiene arm having a number average molecular weightof greater than about 80,000, and with the proviso that the ratio of theweight average molecular weight to the number average molecular weightof the asymmetrical tin-coupled rubbery polymer is within the range ofabout 2 to about 2.5. It should be noted that R₁, R₂, R₃ and R₄ can bealkyl groups because it is possible for the tin halide coupling agent toreact directly with alkyl lithium compounds which are used as thepolymerization initiator. The ratio of silicon containing stars to tincontaining stars will be within the range of about 20:80 to about 80:20in cases where the rubber is coupled with both a silicon and a tincoupling agent.

In most cases, four polydiene arms will be covalently bonded to the tinatom or the silicon atom in the asymmetrical tin-coupled rubberypolymer. In such cases, R₁, R₂, R₃ and R₄ will all be polydiene arms.The asymmetrical tin-coupled rubbery polymer will often contain apolydiene arm of intermediate molecular weight as well as the lowmolecular weight arm and the high molecular weight arm. Suchintermediate molecular weight arms will have a molecular weight that iswithin the range of about 45,000 to about 75,000. It is normallypreferred for the low molecular polydiene arm to have a molecular weightof less than about 30,000 with it being most preferred for the lowmolecular weight arm to have a molecular weight of less than about25,000. It is normally preferred for the high molecular polydiene arm tohave a molecular weight of greater than about 90,000 with it being mostpreferred for the high molecular weight arm to have a molecular weightof greater than about 100,000. The arms of the coupled polymer willtypically be either homopolymers or random copolymers. In other words,the arms of the coupled polymers will normally not be block copolymers.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

EXAMPLE 1

In this experiment, a tin coupled styrene-butadiene rubber was preparedat 70° C. In the procedure used, 2300 g of a silica/alumina/molcularsieve dried premix containing 19.5 weight percent styrene/1,3-butadienemixture in hexanes was charged into a one-gallon (3.8 liters) reactor.The ratio of styrene to 1,3-butadiene was 15:85. After the amount ofimpurity in the premix was determined, 2.4 ml of 1 M solution of TMEDA(N,N,N′,N′-tetramethylethylene-diamine in hexanes), 1.5 ml of 1 Msolution of lithium t-butoxide (in hexanes) and 2.92 ml. of 1.03Msolution of n-butyllithium (in hexanes) were added to the reactor. Thetarget Mn (number averaged molecular weight) was 150,000. Thepolymerization was allowed to proceed at 70° C. for 1.5 hours. The GCanalysis of the residual monomers contained in the polymerizationmixture indicated that most of the monomers were converted to polymer.After a small aliquot of polymer cement was removed from the reactor(for analysis), 1.2 ml. of a 0.6 M solution of tin tetrachloride (inhexanes) was added to the reactor and the coupling reaction was carriedout the same temperature for an hour. At this time, 1.0 phr (parts per100 perts of rubber by weight) of BHT (2,6-di-tert-butyl-4-methylphenol)and 3.0 ml of 1 M solution of TMEDA were added to the reactor toshortstop the polymerization and to stabilize the polymer. Afterevaporating the hexanes, the resulting polymer was dried in a vaccumoven at 50° C. The coupled styrene-butadiene rubber (SBR) produced wasdetermined to have a glass transition temperature (Tg) at −45° C. It wasalso determined to have a microstructure that contained 49 percent1,2-polybutadiene units, 37 percent 1,4-polybutadiene units and 14percent random polystyrene units. The Mooney viscosity (ML-4) at 100° C.for this coupled polymer was also determined to be 108.

The ML-4 for the base polymer (before coupling) was 25. Based on GPCmeasurement, the coupling efficiency was 80%.

EXAMPLE 2

The procedure described in Example 1 was utilized in this example exceptthat lithium t-butoxide solution was added to the polymerization mixturewhen all the monomers were consumed (90 minutes after initiation) andprior to adding the coupling agent. The Tg and microstructure of theresulting coupled SBR are shown in Table 1. The Mooney viscosities ofthe base and coupled polymers are also shown in Table 1. The couplingefficiency was 81%, based on GPC measurement.

COMPARATIVE EXAMPLE 3

The procedure described in Example 1 was utilized in this example exceptthat no lithium t-butoxide solution was used. The Tg and microstructureof the resulting coupled SBR are shown in Table 1. The Mooneyviscosities of the base and coupled polymers are also shown in Table 1.The coupling efficiency was 55%, based on GPC measurement.

TABLE 1 Exam- Tg ML-4 Microstructure (%) Coupling ple (° C.) BaseCoupled 1,2-PBd 1,4-PBd Styrene Efficiency 1 −45 25 108 49 37 14 80 2−44 28 115 50 36 14 81 3 −45 25 85 49 37 14 55

EXAMPLE 4

The tin coupled SBR prepared in this experiment was synthesized in athree-reactor (1 gallon, 2 gallon, 2 gallon) continuous system at 80° C.A premix containing styrene and 1,3-butadiene in hexanes was chargedinto the first polymerization reactor continuously at a rate of 98 gramsper minute. The premix monomer solution containing a ratio of styrene to1,3-butadiene of 18:82 and had a total monomer concentration of 16%.Polymerization was initiated by adding n-butyl lithium (0.6 mmole/100grams of monomer), TMEDA (1 mmole/100 grams monomer) and lithiummentholate (0.5 mmole/100 grams monomer) to the first reactorcontinuously. The resulting polymerization medium containing the liveends was continuously pushed to the second rector (for completing thepolymerization) and then the third reactor where the coupling agent, tintetrachloride, (0.15 mmole/100 grams monomer) was added continuously.The residence time for all reactors was set at one hour to achievecomplete monomer conversion in the second reactor and complete couplingat the third reactor. The polymerization medium was continuously pushedover to a holding tank containing stabilizer and antioxidant. Theresulting polymer cement was then steam stripped and the recovered SBRwas dried in a vented oven at 50° C. The polymer was determined to havea glass transition temperature at −43° C. and have a Mooney ML-4viscosity of 82. The Mooney viscosity of base (uncoupled percurser) was41. It was also determined to have a microstructure that contained 18%random polystyrene units, 41% 1,2-polybutadiene units, and 41%1,4-polybutdiene units.

EXAMPLE 5

The procedure described in Example 4 was utilized in this example exceptthat lithium mentholate solution was formed in-situ by reacting mentholwith n-butyllithium in a catalyst mixer loop before entering the firstreactor. The glass transition temperature (Tg) and microstructure of theresulting coupled styrene-butadiene rubber (SBR) are shown in Table 2.The Mooney viscosities of the base and coupled polymers are also shownin Table 2.

COMPARATIVE EXAMPLE 6

The procedure described in Example 4 was utilized in this example exceptthat no lithium alkoxide solution was used. The glass transitiontemperature and microstructure of the resulting coupledstyrene-butadiene rubber are shown in Table 2. The Mooney viscosities ofthe base and coupled polymers are also shown in Table 2.

TABLE 2 Tg ML-4 Microstructure (%) Example (° C.) Base Coupled 1,2-PBd1,4-PBd Styrene 4 −43 41 82 41 41 18 5 −43 38 82 40 40 18 6 −42 42 68 4241 17

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

1. A process for synthesizing a coupled rubbery polymer which comprises(a) polymerizing at least one conjugated diolefin monomer in thepresence of a stabilized lithium initiator system which is comprised of(1) an alkyl lithium compound selected from the group consisting ofsecondary alkyl lithium compounds and tertiary alkyl lithium compounds,(2) a lithium salt of a saturated aliphatic alcohol, and (3) ahydrocarbon solvent, and (b) terminating the polymerization by theaddition of a coupling agent to produce the coupled rubbery polymer. 2.A process as specified in claim 1 wherein the coupling agent is selectedfrom the group consisting of tin halides and silicon halides.
 3. Aprocess as specified in claim 2 wherein the coupling agent is tintetrachloride.
 4. A process as specified in claim 2 wherein the couplingagent is silicon tetrachloride.
 5. A process as specified in claim 1wherein the coupling agent is a mixture of a tin halide and a siliconhalide, wherein the molar ratio of the tin halide to the silicon halideis within the range of 20:80 to 95:5.
 6. A process as specified in claim5 wherein the molar ratio of the tin halide to the silicon halide iswithin the range of 40:60 to 90:10.
 7. A process as specified in claim 6wherein the tin halide is tin tetrachloride and wherein the siliconhalide is silicon tetra chloride.
 8. A process as specified in claim 2wherein the polymerization is terminated by the addition of a couplingagent in the presence of a polar modifier.
 9. A process as specified inclaim 8 wherein said polar modifier is selected from the groupconsisting of diethyl ether, di-n-propyl ether, diisopropyl ether,di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethylether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether,diethylene glycol diethyl ether, triethylene glycol dimethyl ether,trimethylamine, triethylamine, N,N,N′,N′-tetramethylethylenediamine,N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine, andalkyltetrahydrofurfuryl ethers.
 10. A process as specified in claim 9wherein the coupling is carried out in the absence of sodium alkoxides,cesium alkoxides, rubidium alkoxides, and potassium alkoxides.
 11. Aprocess as specified in claim 8 wherein the molar ratio of the lithiumsalt of the saturated aliphatic alcohol to the polar modifier is withinthe range of about 0.1:1 to 10:1.